# Check your optics skills! Double refraction or trouble refraction

A long cylindrical tube containing water is closed by an equiconvex lens of focal length $$20 \text{ cm}$$ in air. A point source is placed along the axis of the tube outside it at a distance of $$50 cm$$ from the lens. Find the distance from the centre of the lens where the image is formed.

Additional data :- $$n_\text{lens} = 1.5$$ and $$n_\text{water} = \frac{4}{3} \approx 1.33$$

Note: In solving the question you may use the following approximation :- $$0.67 \approx \frac{2}{3}$$.

×